Transponders are used to return a signal on receipt of an interrogating signal. The returned signal can be used merely to indicate that the transponder is present, or can be used to convey further information such as the particular identity of the transponder. A miniature form of such a transponder, sometimes called a tag, can be used to identify goods for security or stock control in say a store, people for security purposes, animals, (e.g. cattle) to ensure positive identification for feeding, treatment, control of disease, pedigree purposes, and so on. The transponder could return other information besides its identity, and could be used to gather information from, say, sensors for returning.
(1) When the information is in the form of digital data, the transmitter requires a clock signal in order to define the bit intervals in the transmitted data. If the data is transmitted synchronously, both the transmitter (in the transponder) and the receiver (in the interrogator) require common clocking information. This could be generated in either the transponder or the interrogator, and transmitted to the other. It is convenient to generate the clocking information in the interrogator, since this simplifies the transponder. The clocking information then requires transmitting to the transponder, but is immediately available for the receiver of the interrogator. Even in the possible alternative of asynchronous transmission from the transponder to the interrogator, it would simplify the transponder if the clocking information is transmitted from the interrogator.
In one prior proposal which uses synchronous data transmission, the clock signal in the transponder is derived by dividing the frequency of the interrogate signal carrier. This has the disadvantage however, that the data rate and the interrogate signal frequency are permanently related. If therefore it were desired in, say, an unusually noisy environment, to reduce the data rate to reduce errors, that would be impossible because the resulting change of frequency in the interrogate signal would prevent it being accepted by the transponder, without retuning the transponder's receiver. That would normally mean redesigning the receiver.
(2) When the information is in the form of digital data, there is a problem of synchronising the interrogator to receive the data in the returned signal. One way to arrange the interrogator to receive data at the right rate, is to transmit clocking information to the transponder in the interrogating signal. The transponder then uses the clocking information to control the data transmission rate.
This, however, still leaves the problem of finding the start of the data. The problem is particularly important if the signal returned by the transponder is in the form of a signal such as a phase shift keying (PSK) signal, in which it is necessary to find the start of a bit in the data.
(3) When the information is in the form of digital data, the signal returned is in the form of a carrier modulated with the information. Naturally, prior art transponders include a modulator for modulating the carrier with stored information.
It is always advantageous to simplify and to reduce costs. However, in the case of a transponder which may be incorporated in, say, an animal's ear tag, reduction of costs and weight have particular relevance.